1. Field of the Invention
The present invention relates generally to pressure regulating devices and methods and specifically to a method and device for regulating the collection of tank vapor within a petroleum storage tank in order to recover the tank vapor for delivery to a gas sales line.
2. Description of the Prior Art
The product from a typical petroleum producing well is first passed to an oil and gas separator, with the liquid petroleum being passed to a petroleum storage tank. The liquified portion of the tank contents are periodically emptied and transported by a tank truck.
While pentane and heavier components of the tank contents are typically liquid, there is always a vapor head within the tank resulting from the lighter components present. Vapor recovery units have been popular for a number of years. These units collect the tank vapor and compress the vapor to the point that it can be delivered to a gas sales line, where the vapor is sold in the gaseous state.
The prior art vapor recovery methods have suffered from a number of deficiencies. Typically, an electric pilot valve on the top of the storage tank was actuated by a predetermined increase in tank vapor pressure. The electrically operated pilot valve closed a bypass control valve located between the scrubber unit and compressor which was used to compress the vapor prior to delivery to the gas sales line. The electrically operated pilot valve posed an electrical hazard because of its proximity to the petroleum vapor. The opening of the bypass valve between the scrubber and compressor caused a decrease in pressure in the conduit leading to the scrubber which produced a refrigeration effect, causing liquids to fall out in the scrubber. This required that liquids be pumped back to the petroleum storage tank, forming an endless loop. Because of the problem of liquid condensation, it was necessary to provide a positive slope on the conduit leading from the storage tank to the scrubber unit. This necessitated that the scrubber and compressor be located immediately adjacent the location of the storage tank. Another problem resulted from the fact that liquids tended to form in the conduit leading to the gas sales line. The orifice meter in the gas sales line did not measure liquids, resulting in a loss in sales.
The object of the present invention is to provide an improved method for controlling tank vapors on a petroleum storage tank.
Another object of the invention is to provide a method for efficiently recovering tank vapors for resale which does not increase the cost or complexity of the presently available systems.
Another object of the invention is to provide a non-electric control system, thereby eliminating fire and explosive hazards.
Another object of the invention is to provide a method for collecting tank vapors which eliminates the tendency of vapors to condense in the conduit connecting the tank to the scrubber unit, whereby the scrubber unit can be remotely located from the petroleum storage tank.
Another object of the invention is to provide an improved pneumatically operated control valve for use in controlling tank vapors on a petroleum storage tank.